Optical transmission of information provides numerous benefits over conventional electromagnetic data transmission including, for example, much higher data capacity and transfer rates with decreased energy consumption.
Optical transmission of data or information employs electromagnetic waves from a spectrum of wavelengths, including, but extending well beyond, visible light, and expressions herein such as “optical”, “light” and related terms are accordingly to be understood in the wider sense of referring to electromagnetic waves within this broader spectrum of wavelengths.
Optical transmission of data or information is typically accomplished by transmission through a dielectric waveguide, such as a fiber optic cable. Light may effectively be modulated in accordance with the data or information to be transported along the dielectric waveguide.
Presently, the majority of optical communication systems presently in operation use direct intensity modulation for conveying digital information. A typical optical telecommunications system is described for example, in the reference Digital Signal Transmission, by C. C. Bissell, and D. A. Chapman, Cambridge Univ. Press, 1992.
Optical transmission systems, due to the many advantages they present, have become widely used for telecommunications. An optical network, such as for example, a local area network (LAN), metropolitan area network (MAN), and wide-area network (WAN), typically include a transmitter at an input end, a receiver at an output end, and a communications medium in between (e.g. an optical fiber). The transmitter's main task is to convert an electrical signal to an optical signal. As the electrical signal enters the transmitter, the binary electrical pulses may be channel and line coded to optimize the integrity of the conveyed data sequence and make it suitable for transmission as, for example, optical pulses.
Typically, a final step for the transmitter is use of an optical pulse generator, which converts the electrically coded pulses into optical pulses. The optical pulse generator may be modeled as a filter with the impulse response related to the desired pulse shape. This type of filter is directed to the actual transmission and detection of individual pulses. This type of filter may, to an extent, be tailored to provide increased bandwidth utilization and for improving receiver sensitivity.
Methods for multiplexing of data and information are also known. For example, digital modulation techniques include variations on neutral, unipolar, polar, NRZ, RZ, and bi-phase. Many optical fiber communications systems use NRZ in conjunction with amplitude on/off modulation for data speeds up to 10 Gb/s.
The more common multiplexing methods include for example, time-division multiplexing (TDM), wavelength-division multiplexing (WDM), and code-division multiplexing (CDM). TDM assigns specific time-slots for every user bits; WDM assigns different wavelengths for different users and/or sub-networks; and CDM assigns each user with a code rather than a time slot and/or specific wavelength.
CDM may, to a certain extent, be viewed as a mixture of TDM and WDM. CDM is becoming the dominant multiplexing method for RF wireless networks and is suggested as the future multiplexing method for optical fiber networks.
When examining TDM, WDM and CDM, each present specific challenges. For example, the main disadvantages with the TDM and WDM methods are: relatively high crosstalk due to optical nonlinearities, the need for temporal and spectral guard bands and a lack of a secure encryption method. A main disadvantage with CDM is the inefficient use of the bandwidth, in addition to some of the disadvantages associated with TDM and WDM.
A number of systems have attempted to provide methods for altering various characteristics of a waveform, such as U.S. Pat. No. 6,826,209 to Morita et al. (“the '209 patent”), which discloses an ultra-broadband, variable and multiple wavelength, waveform shaping apparatus. A light pulse generator enables a fundamental wave light pulse to bring about a self-phase modulation effect, which results in expansion of the spectrum, or causes an induced phase modulation effect between the fundamental wave pulse and the pulse generated by a nonlinear phenomenon that takes place using the fundamental wave pulse. However, the '209 patent fails to teach a system or method that addresses the problems associated with TDM, WDM and CDM, in particular, the problems associated with third-order nonlinearities.
U.S. Pat. No. 6,778,730 to Hironishi (“the '730 patent”) also discloses an optical signal processing device which provides a stable temporal order to the modulation-phases of a plurality of optical signals, the system including an optical demultiplexer and an optical multiplexer for adaptation to WDM (wavelength division multiplexing). However, while the '730 patent may be adapted for use with WDM, the '730 patent fails to teach a system or method that addresses third-order nonlinearity problems in optical transmission systems.
U.S. Pat. No. 5,682,262 to Wefers et al. (“the '262 patent”) still further discloses a method and device for shaping both the temporal and spatial profiles of an input optical pulse to generate an output optical waveform. Waveforms generated with the pulse-shaping device have spatial profiles which either match the pattern imparted by a mask on the optical field (i.e., “shadow imaging”) or are the Fourier transform of the pattern (i.e., “Fourier imaging”). However, the '262 patent fails to teach any kind of system or method that addresses third-order nonlinearity problems in optical transmission systems.
Still another challenge facing the optical transmission industry today is effective encryption of data transmitted via an optical medium. While various encryption methods are known, typically the methods include encryption of the data prior to conversion to an optical signal. One system that has attempted to deal with this challenge is U.S. Published Patent Application No. 2004/0081471 to Lee (“the '0081471 application”), which discloses a method of transmitting data in a dense mode wavelength division multiplex optical system and generally includes the steps of: selectively combining data from a plurality of data channels in a corresponding plurality of optical channels in accordance with an encryption key, transmitting the plurality of optical channels, receiving the plurality of optical channels, and selectively de-combining the data from the plurality of optical channels to receive the plurality of data channels in accordance with the encryption key. However, as with any data encryption system, a determined individual with enough computer power may break the encryption method taught in the '0081471 application.